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Description
The figures in the tables below are reference waveforms collected from known-good vehicles and faulted vehicles. Each figure is accompanied by a description.
Starting - Diesel Engines
| Normal |
Abnormal |
 Courtesy of GENERAL MOTORS COMPANY
- 8 Cylinder Engine
- 4 Cylinder Engine
- 6 Cylinder Engine
|
 Courtesy of GENERAL MOTORS COMPANY
- 3 Cylinder Engine - Low Compression
- Low Compression - The waveform repeats every 3 compression strokes, indicating a current and repeatable compression fault.
- 8 Cylinder Engine - High Starter Load
- Exhaust Valve = Stuck Closed - The waveform repeats every 8 compression strokes, indicating a current and repeatable compression fault. When one cylinder displays double the relative compression, this is an indication of a stuck closed exhaust valve on the companion cylinder.
|
NOTE:
- An engine with little to no compression variance between cylinders will have waveform valleys that are even.
- If the engine compression is equally low across all cylinders, the oscilloscope pattern will appear good.
- The total amplitude (range from highest to lowest point of the waveform) is NOT an indicator of good compression.
Operating Conditions:
- Fuel System - Disabled
- {Automatic Transmission} Apply & Hold - Brake Pedal
{Manual Transmission} Apply & Hold - Clutch Pedal
- Engine - Cranking - For 5 - 10 s
|
Starting - Gasoline Engines
| Normal |
Abnormal |
 Courtesy of GENERAL MOTORS COMPANY
- 8 Cylinder Engine
- 6 Cylinder Engine
- 4 Cylinder Engine
- 3 Cylinder Engine
|
Courtesy of GENERAL MOTORS COMPANY
- 3 Cylinder Engine - Low Compression
- Low Compression - The waveform repeats every 3 compression strokes, indicating a current and repeatable compression fault.
- 8 Cylinder Engine - High Starter Load
- Exhaust Valve = Stuck Closed - The waveform repeats every 8 compression strokes, indicating a current and repeatable compression fault. When one cylinder displays double the relative compression, this is an indication of a stuck closed exhaust valve on the companion cylinder.
|
NOTE:
- An engine with little to no compression variance between cylinders will have waveform valleys that are even.
- If the engine compression is equally low across all cylinders, the oscilloscope pattern will appear good.
- The total amplitude (range from highest to lowest point of the waveform) is NOT an indicator of good compression.
Operating Conditions:
- Fully Applied - Accelerator Pedal @ Wide Open Throttle
- {Automatic Transmission} Apply & Hold - Brake Pedal
{Manual Transmission} Apply & Hold - Clutch Pedal
- Engine - Cranking - For 5 - 10 s
|